This invention relates to a solid-state sensor for the detection of halogenated compounds, such as chloropentafluorobenzene, chlorobenzene, trichloroethylene, dichloromethane, chloroform, chloroethyl ethyl sulfide, dichlorodiethyl sulfide, bromobenzene, bromopropane, iodobenzene or iodopropane.
Many halogenated compounds, especially chlorinated hydrocarbons, are believed to be harmful to human health either through direct exposure (potential carcinogenicity) or indirectly through their adverse effect on the ultraviolet-absorbing stratospheric ozone layer. Sixteen chlorinated hydrocarbons are among the 25 organic compounds that were recently added by the Environmental Protection Agency to the list of chemicals that are to be regulated as toxic wastes under the Resource Conservation and Recovery Act. Rules have also been issued for a 50% reduction in the production and importation of chlorofluorocarbons by the year 1998.
To achieve the required controls, it will be necessary to resort to relatively inexpensive yet well-functioning halogenated compound monitors. It is an object of this invention to provide a sensor that is relatively inexpensive and portable and that can reliably detect various halogenated compounds.
Of the few sensors that have been developed for the detection of halogenated compounds, each has serious shortcomings. A chlorinated hydrocarbon gas sensor consisting of a ZnO-based semiconductor with vanadium, molybdenum and alumina catalysts was reported by M. Shiratori, M. Katsura, and T. Tsuchiya in the Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, T. Seiyama et al., editors (Elsevier, N.Y., 1983), pp. 119-124. Another sensitive sensor system for chlorinated hydrocarbons was reported by J. Unwin and P. T. Walsh in Sensors and Actuators, 18:45 (1989). This system decomposes the chlorinated compounds over a heated platinum coil and measures the changes in the electrical conductivity of a lead phthalocyanine film that is exposed to the decomposition products. Even earlier, Stetter et al. (Sensors and Actuators, 6:269-288 (1984)) reported the detection of such compounds by room-temperature electrochemical (amperometric) sensors following exposure to a heated noble-metal filament. The problem with all of these sensing systems is that they also respond to certain halogen-free compounds, such as methane, ethanol, benzene, hexane or nitrogen dioxide.
There are also gas chromatographic detectors for chlorinated hydrocarbons that are based on Hall conductivity or electron capture. These systems are complex, expensive, and sensitive to interferences.
It is therefore another object of this invention to provide a sensor that is relatively simple, inexpensive, and that can selectively detect halogenated compounds in the presence of potentially interfering substances.
A solid-state sensor, disclosed by J. C. Loh and C. Lu in U.S. Pat. No. 3,751,968, dated Aug. 14, 1973, was claimed to be capable of detecting dichlorodifluoromethane in a concentration as low as 20 ppbv (parts per billion by volume). The same sensor was also intended for the detection of other chlorofluorocarbons, as well as of sulfur hexafluoride, chloroform, and carbon tetrachloride. This sensor is formed of a glass-ceramic mixture of sodium or lithium silicate, lanthanum oxide, and lanthanum fluoride in a preferred molar ratio of La.sub.2 O.sub.3.(1-2)LaF.sub.3.(3-4)Na.sub.2 SiO.sub.3. The preparation of this sensor involves the formation of a surface depletion layer through application of a "biasing D.C. voltage of 1-10 volts" at a temperature above 550.degree. C., preferably 600.degree. C., for approximately 24-48 hours. No information is disclosed in the patent about the performance of this sensor. However, in view of the wide range of preferred molar ratios, one would expect the performance to vary widely from unit to unit.
It is therefore still another object of this invention to provide a sensor that can be prepared in a relatively short time, preferably in not more than about eight hours, and that has well-defined and reproducible performance characteristics.
Other objects of the invention will become obvious to professionals in various fields, such as industrial hygiene or pollution monitoring, following perusal of the complete specification.